1. Field of the Invention
This invention relates to a fuel control system for an internal combustion engine, and more particularly to a fuel control system for a fuel injection type internal combustion engine.
2. Description of the Prior Art
In a fuel injection type internal combustion engine, a basic quantity of fuel to provide a desired air-fuel ratio is calculated according to the quantity of intake air for each cycle and the fuel is injected into the intake system of the engine in the basic quantity for each cycle.
However this method of feeding fuel is disadvantageous in the following point. That is, the fuel cannot be sufficiently vaporized and atomized, and a relatively large part of the fuel injected for each cycle adheres to the wall surface of the intake passage and does not enter the combustion chamber though a part of the fuel vaporizes and enters the combustion chamber during the next injection. Accordingly, the quantity of the fuel actually fed to the combustion chamber for each cycle largely deviates from the required quantity, which can deteriorates the operating performance of the engine and can give rise to a problem in emission control.
In Japanese Unexamined Patent Publication No. 58(1983)-8238, there is disclosed a method of controlling the quantity of fuel to be injected in which the quantity of fuel which is actually fed to the engine is determined on the basis of both the direct delivery part and the drawn part, the former being the part of the fuel to be directly delivered to the combustion chamber from the fuel injector and the latter being the part of the fuel which has adhered to the wall surface of the intake passage, and is vaporized and fed to the combustion chamber. In accordance with this method, the quantity of the fuel to be injected is determined taking into account both the direct delivery part and the drawn part, and accordingly the quantity of the fuel actually fed to the combustion chamber for each cycle approximates to the required quantity. (Such correction of the basic fuel injection quantity will be referred to as "the wet correction", hereinbelow.)
However, since the wet correction is based on estimate of the quantities of the direct delivery part and the drawn part, satisfactory result of the wet correction cannot be expected unless the quantities of the direct delivery part and the drawn part can be precisely estimated. The quantity of the drawn part greatly depends upon various factors which govern the degree of evaporation such as the length of the period during which the fuel vaporizes, the temperature of the fuel itself, the temperature of the wall surface, the quantity of the fuel which has been on the wall surface (will be referred to as "the intake-manifold wetting fuel", hereinbelow) and the like. For example, even if the quantity of the intake-manifold wetting fuel is the same, the quantity of the drawn part increases where the period during which the fuel vaporizes is long and more part of the intake-manifold wetting fuel vaporizes. The intake passage is generally formed by casting and cast wall surface is not smooth and has uneveness, and accordingly, a part of the intake-manifold wetting fuel enters the recesses and stays therein. When the quantity of the intake-manifold wetting fuel is small, contact of the fuel in the recesses with airflow passing through the intake passage becomes worse and the degree of evaporation is lowered, and less fuel is directly delivered to the combustion chamber since the injected fuel directly collides against the uneven wall surface. As a result, the quantity of the intake-manifold wetting fuel increases and the quantities of the direct delivery part and the drawn part are reduced.
In the method disclosed in the above identified Japanese unexamined patent publication, none of the factors which govern the degree of evaporation is taken into account, and accordingly, when the actual vaporizing condition differs from the presupposed condition, satisfactory wet correction cannot be effected.